Hi Coony; I would definitly pin the firewall to side of box, at least four pins to each side. What I have been doing to all my planes, is using L shaped alum and mounting it to the inside of the box. Attaching it with two bolts on side and front, rather then using tri-stock. Your local hardware will handle difference sizes of L shape alum. When I pin the firewall and mount my alum--never had any trouble at all. The weight issue using the alum is nota-nothing-comprendee. O.K. I hope this helps